In a conventional subscriber network based on a twisted metallic pair cable, each subscriber is connected to a local telephone exchange or concentrator via said subscriber's own cable pair. A typical length of cable may be several kilometers. If the pair cable is utilized for transmitting digital information by means of DSL (Digital Subscriber Line) modem techniques, the transfer characteristics of the cable limit the highest achievable transmission rate from a few hundred kilobits per second to a few megabits per second, depending primarily on the cable length.
Subsequently the term “central site” represents the local telephone exchange or concentrator site.
Subsequently a DSL modem stands for, in a broader sense, a transmission device, which converts a digital information stream into an analog signal that can be transmitted over a metallic pair cable by utilizing a usable frequency band and which detects a received signal and converts it reliably into the original digital information stream. A metallic pair cable is a very difficult transmission media incurring strong linear amplitude and phase distortion among other impairments. These impairments are typically equalized by using tools offered by digital signal processing, e.g. adaptive line equalization. Several different modulation methods can be used, e.g. DMT modulation (Discrete MultiTone) or QAM (Quadrature Amplitude Modulation). The modulation method utilized and the detailed implementation of the signal processing, for example, are not essential for the operation of the method and system according to the invention.
When utilizing a conventional subscriber network for digital transmission, the signal conversion in the interface of the subscriber cable and the trunk network is typically carried out by DSLAM equipment (Digital Subscriber Line Access Multiplexer). A DSLAM comprises DSL modems, which transform the digital signal into an analog form suitable for transmission over the metallic cable. The DSL modems also receive the signal coming from the far end subscriber transmitter and convert the signal into digital form. Furthermore, the DSLAM multiplexes traffic from several subscribers and transmits the traffic to the trunk network.
In a digital FTTC subscriber network, data streams for several subscribers are multiplexed to an optical fiber and transported nearer to the subscribers over the fiber so that the lengths of the metallic pair cables are substantially shorter as compared to a conventional subscriber network. As a consequence, the digital transmission speed can be increased considerably, because the shorter the cable is, the higher the usable bandwidth. A new problem arises in an FTTC network from the fact that now the DSLAM is placed nearer to the subscribers and away from the central site. The DSLAM of a conventional network is installed at the central site, where it is much easier to arrange the necessary power supply and where the environmental factors (temperature and humidity) crucial for the equipment electronics are easier to control. In an FTTC network topology, the remote DSLAM (subsequently RDSLAM) has to be installed, for example, in a box located at a street corner, where the power supply arrangement is laborious and where the electronics suffer from wide temperature and humidity variations. A substantial portion of the electronics of DSLAM equipment is located in the digital transceivers of the DSL modems and in the circuitry multiplexing the data stream onto the optical fiber. Also, a substantial amount of power consumption is used for supplying said electronics. FIG. 1 represents FTTC network topology based on known technology. An optical fiber 2 connects RDSLAM equipment 3 to equipment 1 at the central site, where the fiber is connected to a switching device of the trunk network, e.g. an ATM switch or an IP router, depending on the transport protocol.
The method described in U.S. Pat. No. 5,526,154 to Pyhalammi succeeds in overcoming some of the problems described above. According to the method of reference, the analog interface of the DSL modems at the central site DSLAM is connected to the system so that the central site equipment converts the modem analog signal into a digital sample sequence. The sample sequence is transmitted through an optical fiber to the far end and converted back into an analog signal, which is then fed into a twisted pair subscriber cable. The same procedure is carried out as well in the reverse transmission direction, from the subscriber towards the central site. Hence the system is entirely symmetrical, and identical equipment is needed at both ends of the optical fiber. Thus the interface between the system and the central site DSL modems connects analog signals in both 15 transmission directions. In another method of reference, the modem signals are multiplexed as analog signals by using analog modulation and demodulation.
In WO03017634 the present inventors introduced a method whereby the equipment at the subscriber end of the optical fiber can be implemented with simple active electronics comprising only the analog parts of the DSL modems and a multiplexer element adapting the analog-to-digital and digital-to-analog converters of the analog parts to the optical fiber.
In the methods of the above-cited references the electronics of RDSLAM equipment are considerably simpler compared to a conventional RDSLAM. In systems according to the cited methods, likewise in a conventional FTTC system, there is still a problem in supplying the operating electric power. The operating power has to be fed to the equipment either from the central site via a power feeding cable or by connecting a power feeding cable from the main system for electricity distribution to the RDSLAM. The installation and maintenance cost is thus increased, primarily because it is not always possible to draw power-feeding cables through the shielding tubes originally intended for optical fibers.
The objective of the present invention is to overcome the problems of prior-art technology and to provide an entirely novel method for establishing an FTTC subscriber network and arranging the operating power feeding for RDSLAM equipment.